There's no minimum or maximum pipe size in NEC 250.52(A)(1) nor is there a material specified, only that it be "metallic".
The clamp thingamajig is pretty creative, too
Many inspectors in this area won't accept a capped pipe as part of the Grounding Electrode, because it may contain only trapped air. As lame as this thing looks, it might have been done in response to a correction notice.
Can a plumber put a hose bibb indoors with no drain?
![[Linked Image]](https://www.electrical-contractor.net/ubb/biggrin.gif)
The clamp thingamajig is pretty creative, too 
